Linen Cloth can be dropped by any humanoids between the levels of 6 to 15, but the most efficient place to farm for Linen Cloth in world of Warcraft would be Sentinel Hill at Westfall, Linen Cloth will drop from Riverpaw Gnolls which respawns incredibly fast. Some Riverpaw Gnolls will spawn before you finish killing the first one. For the Horde faction, durotar is also a good place to farm Linen Cloth, the area below Southfury Watershed for the Razormane Quillboars. they spawn at a decent rate and there is plenty.
Check out the map below for the specific area for where to find linen in WoW.